What are the colored/highlighted edges in edit mode?
Those edges have been marked.
There are actually five types of markings you can apply to edges:  


Red - UV Seam
Blue - Bevel Weight
Purple - Edge Crease
Cyan - Sharp Edge 
Green - Freestyle Edge


You can mark edges yourself from Edit Mode using the Edge menu (Ctrl+E).



Colors in Blender 2.79:


Red - UV seam
Orange - Bevel weight
Pink - Edge crease
Blue - Sharp edge
Green - Freestyle edge